Johansson, Tommy

Abstract [en]

Dusty plasmas is a relatively new area in plasma physics. The field has received increasing attention over the last decade. At the Alfvén-laboratory, KTH, the work on dusty plasmas has recently begun. This Master thesis concerns the diagnostics of a spherical plasma, called a fire ball by some authors, and the effects of dust on it. A dusty plasma is a plasma in which not only positive ions and electrons are present, but also larger particles, dust grains. The size of these grains are typically of the order of mm. The dust particles may be charged by the colliding electrons and ions, through photo ionisation and a number of other mechanisms. The influence of gravitation can often not be neglected. The grains may under certain circumstances be levitated, when the gravitational force and the electromagnetic force are equal in magnitude but opposite in direction. The properties of the plasma may be strongly altered in the presence of high densities of dust. A plasma, by definition, is considered to be electrically quasi-neutral. To retain the quasi-neutrality also for dusty plasmas, the charge density of the dust must be added to the densities of ions and electrons. Dust clouds in a laboratory enviroment have been found to form structures [see for example Morfill and Thomas 1996: Nuomura et al 1998: Samarian and Vaulina 2000]. The term used to describe the ordered state of dust in a plasma, is plasma crystal, which is in some sense similar to crystal structures found in metals. Dusty plasmas are common in space. Dusty plasmas are found in the tails of comets, in planetary rings to name a few. In particular, interest of dusty plasmas in context with the rings of Saturn have been shown [Goertz 1989: Northrop 1992: Brenning 2001]. Dusty plasmas are also important in technological applications, as dust particles are often found to contaminate plasma processes. The objective of this work, was to perform diagnostics on the non-dusty plasma inside and outside the fire ball, which is the name for an anode plasma with spherical shape. In addition effects on the plasma caused by dust was searched for. In the plasma tank, where the experiments were performed, there is also a background plasma present. At the boundary between the fire ball and the background plasma, an electrical double layer were assumed to exist. Experiments were designed to investigate the characteristics of this layer. The shape of the fire ball were seldom perfectly spherical. Two modes of the spherical plasma were observed, one of which was a ball with sharp visual edges, whilst the second had more diffuse edges. The difference between these two modes was of interest.